1. Field of the Invention
The present invention relates to an electrodeless discharge lamp and, more particularly, to a transformer for driving a Class D amplifier for decreasing power consumption in an electrodeless discharge lamp.
2. Description of the Prior Art
In U.S. Pat. No. 4,010,400, there is disclosed an electrodeless discharge lamp of the type including an ionizable medium within a sealed envelope and including at least one particular ionizable gas at a given pressure capable of emitting radiant energy when subjected to a radio frequency field. An electric field having a magnitude sufficient to initiate ionization of the medium to form a radiation emitting discharge is coupled to the medium. Simultaneously, a radio frequency magnetic field for maintaining the ionization is coupled to the medium. If the various parameters of the lamp are properly selected, a high efficiency electrodeless fluorescent lamp is theoretically possible.
It is known to drive the medium by means of an oscillator, which is usually crystal controlled, for generating an output signal at a given radio frequency, an RF amplifier responsive to the oscillator output signal, and an induction coil and a capacitor connected in series and responsive to the output of the amplifier. The coil is positioned in close physical proximity to the medium in the envelope for coupling to the medium the electric field and the magnetic field.
In my copending application Ser. No. 278,888, filed concurrently herewith, and entitled Circuit Means for Efficiently Driving an Electrodeless Discharge Lamp, such application being assigned to Litek International, Inc., the assignee of the present application, there is disclosed circuit means for efficiently driving such an electrodeless discharge lamp. According to the invention described in such patent application, a Class D amplifier circuit is used without a load resistor and in a manner which substantially reduces 1/2CV.sup.2 f losses. A single circuit functions as a filter to reduce RFI and as a matching network to provide an operating voltage to set the light output level.
The preferred embodiment of RF amplifier includes a pair of series connected MOS transistors connected between a source of dc voltage and ground. The gates of the transistors are connected to separate secondary windings of a transformer. The windings are out of phase. The primary of the transformer receives the RF signal from the oscillator. By making the secondary windings out-of-phase windings of a single transformer, the transistors can be driven alternatively on and off, thereby producing a square wave output signal which alternates between the source of dc voltage and ground. This signal is filtered to produce the clean sine wave necessary to drive the discharge.
In such a drive system for a Class D amplifier, power is consumed only during the switching interval. That is, the transistors consume no power when they're either on or off, current only flowing, called the idling current, during the switching interval. If the switching interval can be decreased, the time interval during which the idling current flows will be decreased, as will be the power consumed. Since efficiently converting input energy into output power is essential if an electrodeless fluorescent lamp is to compete effectively with other types of lamps, the reduction in the idling current to a minimum value is essential.
In the past, it has been common practice to wind the primary and secondary windings on a common toroidal, magnetic core which maximizes coupling between the primary and each of the secondaries. However, there exists some inherent capacitance across the gates of each MOS transistor so that there is also mutual coupling between the secondaries. That is, the turning on of one transistor is affected by the turning off of the other and vice-versa because of the mutual charge that flows between the two input capacitances. In the past, this has created a lower limit on the switching time.